New York (CNN/Money) - Thirty years ago, many Americans might have thought of "organic food" as something they served for supper at the commune. Today, thanks to the hoopla about the health benefits of organic food, even the most staid people are adding it to their daily menus.

Between 1980 and 2000, Americans increased their organic foods purchases a hundredfold. This year, according to the Department of Agriculture (USDA), sales will reach $13 billion.

Sporting a gaudy average annual growth rate of more than 20 percent per year for the past 10 years, organic food production is the fastest growing sector of U.S. agriculture.

If anything is holding the sales of organic food sales back, however, it's the expense. According to a survey by eBrain Market Research, 67 percent of Americans said price was a barrier to buying organic foods.

Organic foods cost more because they're grown without artificial fertilizers, and they employ natural pest and weed controls, promoting biodiversity and improving soils. All that is much more labor intensive -- and expensive -- than conventional farming.

Just how much more buying organic would add to your food bill?

For an answer, the Organic Trade Association (OTA) recruited the help of environmental activist and organic food advocate, Sandra Steingraber.

Steingraber teaches at Ithaca College and is the author of "Living Downstream: An Ecologist Looks at Cancer and the Environment." Her relationship with organic foods began in early childhood when her father, influenced by Rachel Carson's "Silent Spring," made the family garden totally organic.

She prepared, from scratch, two pizzas, one using organic ingredients and one using conventional ones.

Charting the pie

To make a pizza, Steingraber needed a half-cup of virgin olive oil, one-and-a-half cups of whole wheat flour and an equal amount of all-purpose flour, a can of tomato paste, two medium-sized tomatoes, three cloves of garlic, and a cup of grated mozzarella cheese, all in both organic and conventional versions, as well as water, sugar, yeast, and salt.

She found organic fresh tomatoes (69 cents vs. $1.20) and garlic (7 cents vs. 12 cents) selling for less than conventional counterparts, but all the other organic pie ingredients cost more. Organic mozzarella ($1.98 vs. 80 cents), for example, was 147 percent more than conventional cheese, and a small can of tomato paste cost almost twice as much.

Totaling it all up, an organic pizza cost $6.45 and a conventional one $4.50, a 43 percent difference. Organic food advocates, however, say that's only half the story.

Steingraber points out that the checkout price of organic food represents practically the full cost of its production. On the other hand, conventionally grown food produces far more of what economists call "externalities" -- costs of an activity borne by others.

Cornell University professor David Pimentel estimates the indirect cost of pesticide use in the United States at $10 billion annually. About a billion of that is to clean ground and surface water, but the lion's share of the cost comes under the heading of "public health."

Where to buy it

The USDA, which regulates the claims of organic food producers, breaks down organic foods into four categories:

100% Organic: Products cannot contain any non-organic ingredients.

Organic: 95 percent of the product's ingredients must be organic.

Made with Organic Ingredients: At least 70 percent of the product's ingredients must be organic.

Some Organic Ingredients: Products containing less than 70 percent organic may list them individually.

More than 20,000 natural food stores in the United States, including chains such as Whole Foods Market, Wild Oats, and Real Food, now stock extensive assortments of organic products. Supermarkets, such as Albertsons, A&P, and Kroger, display whole aisles of organic goods.

Farmers markets and farm stands are also great places to find organic food, and produce from there tends to be fresher and more flavorful.

To get started buying organic foods, the OTA advises you to concentrate at first on buying organic alternatives to the foods your family eats most.

The trade group also suggests you reduce your exposure to "persistent organic pollutants" (POPs) by switching to the organic versions of butter, cantaloupe, cucumbers/pickles, meatloaf, peanuts, popcorn, radishes, spinach, and squash. These foods produced through conventional means are the most likely to be contaminated with POPs.

Finally, ask for the foods you want. Store managers can special-order most organic foods and frequent questions will encourage stores to stock even more organic items.